It is standard to sterilize containers such as prefilled syringes having plungers in an autoclave that forms a pressurizable chamber around the containers being sterilized and that is pressurized with hot air and/or saturated steam. The chamber is maintained at a superatmospheric pressure of about 1.1 bar and the containers are heated to about 120° C. for 20 to 60 minutes to kill any bacteria.
As the syringes are being brought from room temperature to the desired high autoclave temperature, the contents, typically both liquid and gas, expand somewhat. This can have the deleterious consequence of moving the plunger from its starting position. When the syringes are subsequently cooled, static friction normally prevents the contracting gas/liquid contents from pulling the plunger back to the starting position.
This displacement of the plunger has several disadvantages. First of all, if the contents of the syringe are left under subatmospheric pressure, it is possible that the syringe will draw in nonsterile air, contaminating its contents prior to use. Furthermore when the syringes are to be fitted with plunger stems after autoclaving, the machine that mounts the plunger stems, which project rearward out of the syringe body, can damage the plunger, stem, and/or syringe body since the plunger will not be in the position the stem-installing device is set for.
Hence it is known to control the pressurization of the autoclave by means of a computer program that takes into account the composition of the contents of syringes and their coefficients of thermal expansion. This program is used to increase the pressure in the autoclave around the syringes as they are heated, with the aim of maintaining the pressure inside the syringes the same as the pressure outside so that the plungers do not shift. Similarly as the containers being autoclaved cool down, the program reduces the autoclave pressure, thereby also preventing the plungers from shifting.
Such a solution is technically very complex. As the composition of the liquid in the syringes changes, the program must be corrected, as different liquids volatilize at different temperatures and to different extents. Similarly different plungers and syringe bodies have different coefficients of frictions, requiring more or less sensitivity in the calculation. Hence it is necessary to reprogram each time the syringe contents being autoclaved changes, and it is absolutely impossible to simultaneously autoclave syringes holding different substances. What is more, different syringes conduct heat differently and the contents of the syringes have different heat capacities and abilities to conduct heat, further complicating calculation of the pressure/temperature curve.
The result is that programming the increase and decrease in pressure in the autoclave is a hit and miss operation, typically done more empirically or experimentally than in accordance with any manageable algorithm. What is more, within a given batch of syringes it is possible for there to be some variation as to air content and the like. The result is that under the best of circumstances, one can count on the plungers moving somewhat. Even when the plungers end up in the starting position, if they have moved significantly, the result can be deposits of the medicament between the plunger and the inner side walls of the syringe or even on exposed inner surfaces of the syringe.